It is estimated that in the automotive and associated coatings industries, approximately 20% to 40% of the total volume of paint sprayed is overspray. The overspray is removed from the paint spray booth by air currents which are scrubbed by recirculating water. The oversprayed paint particles become entrained in the spray booth water which, if untreated, may cause plugging of piping and spray nozzles thus reducing the operating efficiency of the spray booth and increasing the voc emissions. Present paint spray booth detackification treatments are based upon clays (e.g. hectorite or bentonite) amphoteric metals (e.g. sodium zincate) and polymers (e.g. diallyldiamethyl ammonium chloride, melamine aldehydes, etc.) or a combination of these.
While the majority of paints currently used in the market are organic solvent based, in an attempt to lower voc emissions from the painting operation, the use of waterborne coatings is becoming increasingly more common. Waterborne paints are resin suspensions stabilized by a surfactant matrix which upon incorporation into the paint spray booth water disperse and/or emulsify resulting in an increase in both suspended and dissolved solids. These solids must be coagulated and removed from the spray booth water in order to maintain an effective painting operation. These solids do not, however, require detackification as required by organic solvent based paints.
There are, however, some paint spray booth operations wherein both organic solvent based paints and waterborne paints are sprayed into the same booth. Due to major differences in the formulations of waterborne paints and solvent based paints, separate paint detackification agents are necessary to treat the respective paint particles in the spray booth water. Consequently it would represent a major benefit if a single water treating agent could act not only as a coagulant but could also act as a detackifier or optionally if the coagulant could be used in combination with a detackifier without decreasing the performance of the detackifier.
Various aluminum containing compounds have been used as flocculating and coagulating agents in water treatment systems. For example, aluminum sulfate, also known in the trade as alum, has been used extensively as a coagulant in paint spray booth water. However, the use of alum has numerous disadvantages, in particular, it produces a high level of residual, soluble aluminum compounds which require additional treatment prior to discharge. In addition to alum, polyaluminum chloride has been considered for use as a coagulant aid in paint spray booth water. However, this material was found to be ineffective for coagulating water-borne paints. It was therefore surprising and unexpected that another polyaluminum salt was found to effectively coagulate waterborne paints in paint spray booth recirculating water.